The present invention relates to a solvent recovery system and, more particularly, to a solvent recovery system for recovering the organic solvents used during the coating of video tape and like materials.
Various magnetic tapes, such as video tape and audio tape, are manufactured by coating a mixture of metal oxide powder, organic solvent, and binder on a plastic film. The solvent (e.g., a mixture of methyl ethyl detone, toluene, methyl isobutyl ketone is then evaporated in an oven in order to leave a mixture of the metal oxide powder and binder on the surface of the film. Solvent reclamation is a necessity for both economic and environmental reasons.
A preferred oven for the separation of solvent from the binder and iron oxide is a non-contact dryer oven, such as an inert flotation dryer oven. Unlike conventional ovens which use rollers to transport the film, the flotation dryer over allows the film to pass through it without making any physical contact. Such an oven is typically equipped with inlet and outlet isolation curtains for maintaining an inert gas inside the dryer, preferably with a minimal oxygen intrusionn of 4% or less. For reasons of safety and economy, nitrogen is commonly used as the inert gas, and the inert gas will hereinafter be referred to as nitrogen, although it is to be understood that other inert gases may be used in addition to or instead of nitrogen. Because the process utilizes inert gas in the drying oven instead of air, there is less danger of explosion or fire in the oven. As a result, a lower gas velocity may be employed without danger of exceeding the lower explosive limit of the solvent concentration in the vapor stream.
The coated film is pulled by a coating line through the flotation oven, the film being supported on a cushion of nitrogen vapor, typically induced by gas nozzles disposed both below and above the film. The nitrogen vapor pressure of the many gas nozzles spaced above and below the travel path of the film is adjusted to maintain the desired film path. Typically the oven includes several zones, each zone being maintained at a different temperature, to permit a controlled increase in temperature through the oven in the direction of the travel path so that the speed of solvent evaporation is controlled.
A blower, such as a fixed displacement blower, removes the vapor stream including the nitrogen vapor and the solvent vapor from the oven. The discharged vapor stream is then passed through a solvent recovery means in order to condense the solvent vapor in the vapor stream and separately discharge the nitrogen vapor and liquid solvent. The liquid solvent is typically stored for reuse in the mixture used to coat the film, and the nitrogen vapor is returned to the inert gas nozzles in the oven, thereby effectively recycling both the nitrogen and the solvent. A preferred solvent recovery means is the condensation-type wherein the solvent vapor is cooled below its liquification temprature in a series of heat exchangers which operate at temperatures above the liquification temperature of the nitrogen vapor, so that the nitrogen remains in the vapor phase while the solvent enters the liquid phase, thereby facilitating separation of the two as different phases. Where the condensation-type solvent recovery means utilizes ambient water in a heat-exchange relationship with the vapor stream in order to condense the solvent vapor, the efficacy of the solvent recovery means varies greatly with the temperature of the cooling water. As ambient cooling water may vary in temperature widely, dependent for example upon the season of the year, the capacity of the solvent recovery means also varies considerably.
Accordingly, it is an object of the present invention to provide a solvent recovery system utilizing a flotation-type coating oven and a condensation-type solvent recovery means which does not rely on ambient water for condensing the solvent vapor within the solvent recovery means.
Another object is to provide such a system which is simple, inexpensive and efficient to operate.